The present invention relates to a high strength bolt and a method of manufacturing the high strength bolt, and more particularly, to a nonheat-refined high strength bolt excellent in toughness as well as tool life and having a tensile strength not smaller than 80 kg f/mm.sup.2 and a method of manufacturing the same which method is characterized by austenitizing a low or medium carbon-manganese steel wire material, and then subjecting the austenitic steel wire material to isothermal transformation, wire drawing and cold forging successively, without quenching and tempering steps.
In general, a high strength bolt having a tensile strength not smaller than 80 kgf/mm.sup.2 is manufactured by a method wherein a medium carbon steel wire rod is subjected to spheroidizing annealing and wire drawing, followed by bolt forming by means of cold forging and is then subjected to hardening and tempering, thereby to provide the bolt with a necessary strength and toughness, as shown in U.S. Pat. No. 3,532,560 dated Oct. 6, 1970.
Attention has recently been paid to a method by which a steel product having a high strength and toughness can be manufactured without requiring hardening and tempering after the cold forging, as described in U.S. Pat. No. 3,340,102 dated Sept. 5, 1967. However, this method can not practically apply to the mass production of bolts because of the necessity to effect mechanical deformation at a relatively high temperature range defined by the metastable austenite region of a material to be worked.